The present invention relates to a hybrid structure for a flexible pipe for great depths, reinforced with armouring layers and designed for the transportation of effluent under pressure. The structure according to the present invention is particularly well suited to flexible pipes of the flow line type, that is to say flexible pipes paid out from a ship to be laid generally on the seabed and connected to the underwater installations and which work essentially in a static situation.
The flexible pipes used at sea are subjected to various types of external stressing.
The flexible pipes designed to be used in fairly shallow or moderately shallow water (typically between 100 and 800 m) have structures which can vary widely, depending on the conditions of use.
The flexible pipes most widely used in oil production are generally of the unbonded type, in which the various successive and distinct layers have a certain freedom to move with respect to each other, and they comprise, from the inside outwards, a carcass consisting, for example, of a clipped fabric tape which serves to prevent the pipe from being crushed under the external pressure, an internal sheath made of polymer, a pressure vault consisting of at least one shaped interlocked wire wound in a short-pitch helix, so-called tension armours whose lay angle, measured along the longitudinal axis of the pipe, is less than 55.degree. and finally an outer-sheath made of polymer. Such a flexible pipe is denoted as being a "rough-bore" flexible pipe.
When a flexible pipe comprises, from the inside outwards, an internal sheath, a pressure vault consisting of shaped interlocked wires wound with a short pitch and intended to withstand the hoop stresses caused by the flow of effluent through the flexible pipe, an anti-collapse sheath, one or more tensile armours wound around the anti-collapse sheath and an outer-sheath made of polymer, it is known as a smooth-bore.
The elements which make up these various structures are defined in documents API 17B and 17J compiled by the American Petroleum Institute under the title "Recommended Practice for Flexible Pipe".
In an alternative form, the flexible pipe has no pressure vault and the armour layers are spiral-wound with reverse pitch with lay angles of close to 55.degree.. In this case, the internal and external pressures and the tensile forces are exerted or supported by these armour layers; such a pipe is said to be balanced.
Examples of structures of flexible pipes are described, for example, in FR-A-2,619,193 and FR-A-2,557,254.
Proposed in FR-A-2,619,193 is a flexible pipe whose dimensional variations, particularly in the axial direction, can be controlled in such a way that dimensional stability or controlled shortening can be achieved when the internal pressure is raised.
In FR-A-2,557,254, the flexible pipe is designed not to exhibit an appreciable variation in length when subjected to an internal pressure with a "direct end cap effect" which is induced by the pressure within the flexible pipe.
In the flexible pipes currently available, the armouring layers are designed to withstand all the stresses they experience during manufacture, transportation, laying, service and recovery. In particular, they need to withstand the tensile stresses during the laying of the flexible pipe, which stresses are induced by the total weight of the flexible pipe, and the pressure stresses so as to be able to withstand internal and external pressures. The hoop stresses are generally absorbed by a pressure vault when there is one in the flexible pipe. This being the case, designers are looking for a structural compromise without concerning themselves as to whether each of the constituent elements of the structure plays a part in one or more functions, as is the case, for example, of armouring layers at 55.degree. which react the tensile forces just as well as the internal and external pressures experienced by the said flexible pipes.
The external stresses, also known as loading scenarios by the specialists, may be classified into two main categories.
The first category relates to the laying of the flexible pipe from the surface onto the seabed. In this scenario, the weight of the said submerged pipe develops a tensile stress.
The second category relates to the conditions of use of the said pipe that is to say of the pipe laid on the seabed after it has been connected to the seabed equipment. In this case, the stresses experienced by the flexible pipe are essentially due to the differences in pressure between the internal pressure P.sub.int inside the flexible pipe and the external pressure P.sub.ext exerted on the said flexible pipe and equal to the pressure of the column of water that lies above the said flexible pipe.
When the difference .DELTA.P=(P.sub.int -P.sub.ext) is positive, the stresses induced in the flexible pipe are radial and longitudinal, the two stresses being considered as positive because they are directed towards the outside of the pipe. The radial stress translates into a radial expansion, while the longitudinal stress translates into a lengthening of the flexible pipe.
When the difference .DELTA.P=(P.sub.int -P.sub.ext) is negative which essentially corresponds to an empty pipe on the seabed, the stresses are considered as being negative because they are directed towards the inside of the pipe. The radial stress translates into a compression and the longitudinal stress translates into a shortening of the said flexible pipe.
The longitudinal stresses in both instances where the difference .DELTA.P is positive or negative are tensile stresses.
The end cap effect T induced in a flexible pipe depends, among other things, on the pressure difference .DELTA.P=(P.sub.int -P.sub.ext) and on the internal and external radii of the said pipe.
When T is positive, the end cap effect is said to be a direct end cap effect.
When T is negative, the end cap effect is said to be a reverse end cap effect.
Down to a certain depth, the reverse end cap effect has little damaging effect on the pipe. By contrast, for greater depths of water, greater than 1200 m for example, the reverse end cap effect may have serious consequences which may go as far as to damage the flexible pipe. This is because when T is highly negative, the axial compressive stresses in the armouring layers become high and the diameter of the helical winding then tends to increase more than might be wished. However, since the armouring layers are restrained by the outer-sheath and possibly by a banding around the said armouring layers, the radial expansion is restricted between acceptable values. However, above and beyond a certain stress value, there may be local plastic deformation of armour layer wires, the consequence of which is irreversible damage to the flexible pipe.
When the outer sheath bursts for whatever reason, the pressure in the annulus increases and becomes equal to the external pressure exerted on the flexible pipe. At a depth of 1800 m, the external pressure is equal to about 180 bar. The armouring layers which experience this external pressure tend to twist. If now, the internal pressure, which was, for example, 300 bar, drops to 1 bar, the armouring layers being expanded on account of the compression due to the 180-bar external pressure, they may deform into a "birdcage".
In application FR-A-2,756,605, there is described a flexible pipe which overcomes the drawbacks of the flexible pipes described in FR-A-2,458,022, WO-96/17198 or GB-A-1,486,445 and which comprises, from the inside outwards, a corrugated metal internal tube, a pressure vault, a polymer anti-collapse sheath, two armour layers wound with a lay angle of less than 55.degree. with respect to the longitudinal axis of the pipe, a reinforcing fabric tape and finally an outer-sheath. The internal tube or liner has corrugations which are spaced apart along the entire length and which face the interior surface of the pressure vault.
The flexible pipe described in this published application constitutes a satisfactory solution as far as H.sub.2 S corrosion is concerned, because the liner constitutes an effective barrier against the diffusion of gases for two-phase effluents of the crude oil (or "life crude") type. However, it does not solve the issue of the reverse end cap effect, because neither the anti-collapse sheath nor the reinforcing sheath can substantially reduce the consequences of a reverse end cap effect for great depths.